Instructions
Identify what it is you're looking for. If you have a specific item or type of item that you're looking for, it gets easier to identify what it is you're looking at.
Research that item specifically. Some things you should pay attention to are: colors of the items, markings that are unique to it, features the item boasts and where it was manufactured. If you're looking for Soviet military items, but the item was made in China, it's important to know whether or not the Soviets did business with China or if the item you're looking at is a fake.
Look at pictures of the item on websites. Auction sites and military surplus sites should have pictures of the item that you're purchasing, and it pays to examine it in detail before you decide it's legitimate. If all of the details match up with what your research tells you to expect, and the price matches an item of actual historical value, then it's probably legitimate.